Pores, inclusions and slag inclusions and preventive measures

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(1) Porosity: During welding, pores are formed when gases present in the molten pool fail to escape during solidification and remain behind, as shown in Figure 1. Porosity is a common welding defect, which is divided into internal porosity and external porosity in the weld. Stomata come in various shapes such as circular, oval, worm-like, needle-shaped, and clustered. The presence of stomata not only affects the density of the weld but also reduces its effective area, thereby lowering the mechanical properties of the weld. Causes: Contaminants such as oil, rust, and moisture are present on the surface of the weldment and at the groove; the coating on the welding rod is damp and has not been dried before use; the welding current is too high or the welding speed is too fast; the arc is too long or misdirected, resulting in poor protection of the molten pool and allowing air to enter it; the welding current is so high that the welding rod turns red and its coating falls off prematurely, thereby losing its protective function; improper welding technique, such as moving the welding rod too quickly when ending the arc, can lead to shrinkage cavities, while incorrect starting of the arc at the joint can result in numerous pores.                                           Measures to prevent porosity: Before welding, remove all oil, rust, and moisture from within a range of 20–30 mm on both sides of the groove. Bake the electrodes strictly according to the temperature and time specified in the electrode instructions. Select the appropriate welding process parameters and operate correctly; try to use short-arc welding, and install wind protection measures when working outdoors. It is not allowed to use defective electrodes, such as those with corroded cores, cracked or peeling coatings, or excessive eccentricity. (2) Inclusions and slag inclusions: Inclusions are non-metallic particles and oxides produced by metallurgical reactions that remain in the weld metal. Slag inclusions are slag that remains in the weld. As shown in Figure 2, slag inclusions can be divided into two types: punctate slag inclusions and linear slag inclusions. Slag inclusions reduce the effective cross-sectional area of the weld, thereby lowering its mechanical properties. Additionally, these inclusions can cause stress concentration, making the welded structure prone to failure under load. Figure 2 Causes of inclusions in welds: Inadequate removal of slag between layers during welding, too low welding current, too fast welding speed, improper operation during welding; inappropriate chemical composition matching between welding materials and base material; unsuitable design and preparation of the groove, etc. Preventive measures: Select electrodes with good slag removal properties; carefully remove interlayer slag; appropriately select welding process parameters; adjust the electrode angle and welding technique